Optical scanner is a widely used computer peripheral device for image process. It has a mirror means and a charged couple device (CCD) for scanning and capturing document image, and converting the image to digital signals for computer process.
FIG. 1 illustrates a conventional scanner 1 which generally includes a casing 10, an optical path means 12, a driving means 14 and a motor 16.
The casing 10 is hollow and contains a scanning path inside such as a guide rod or rail (not shown in the figure) to enable the optical path means 12 to move along to and fro for scanning. Above the casing 10, there is a transparent document board 102 for holding a scanning document 2. Above the casing 10, there is further a cover 104 pivotally hinged with one end of the casing to cover the document 2 during scanning operation.
The optical path means 12 includes a light source 122 which may be a lamp tube, a plurality of mirror sets 124 which constitute a plurality of reflection mirrors 1241 and a lens 1243, and a CCD 126. The light source 122 projects light ray to the document 2. Document image then reflect to the reflection mirrors 1241 which direct image light to the CCD 126. The CCD 126 captures and converts image light to digital signals which are sent to computer for process.
The driving means 14 includes a close loop belt 142 wound around two pulleys 144a and 144b located at two ends inside the casing 10. The belt 142 engages with the optical path means 12. The motor 16 has an output shaft engages with the pulley 144a. When the motor 16 is activated, it drives the pulley 144a which in turn drives the belt 142 and the optical path means 12 moving to and fro within the casing 10 for scanning the document 2.
The two dimensional scanning on the document 2 is composed of multiple one dimensional straight lines. The optical path means 12 reads one line at a time then is moved to read a next line until all lines on the document are read.
The resolution of the scanner 1 depends on the image density read by the optical path means 12. DPI (dot per inch) defines the scanner resolution standard. It means the image number read by the scanner in an inch of document.
In conventional scanner, the motor rotation speed is constant. Hence the image number read by the optical path means is constant in a fixed time period. The resolution of the scanner thus is fixed and unadjustable. To adjust the resolution, the motor speed should be changed to change the scanning speed of the optical path means 12. Its cost is higher and the adjusting process is complicated.